Light emitting element array and optical transmission device

ABSTRACT

A light emitting element array includes plural light emitting elements connected in parallel to each other by a wiring connected to a terminal that supplies a current. Each of the light emitting elements is disposed at a position of a predetermined path length along a path of the current flowing from the terminal through the wiring. The plural light emitting elements include, in a mixed form, one or more first light emitting elements each having a non-shielded light emission aperture and one or more second light emitting elements each having a shielded light emission aperture. At least one of the first light emitting elements is disposed at a position of the longest path length. At least one of the second light emitting elements is disposed at a position of the shortest path length.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 fromJapanese Patent Application No. 2016-223546 filed Nov. 16, 2016.

BACKGROUND Technical Field

The present invention relates to a light emitting element array and anoptical transmission device.

SUMMARY

According to an aspect of the invention, a light emitting element arrayincludes plural light emitting elements connected in parallel to eachother by a wiring connected to a terminal that supplies a current. Eachof the light emitting elements is disposed at a position of apredetermined path length along a path of the current flowing from theterminal through the wiring. The plural light emitting elements include,in a mixed form, one or more first light emitting elements each having anon-shielded light emission aperture and one or more second lightemitting elements each having a shielded light emission aperture. Atleast one of the first light emitting elements is disposed at a positionof the longest path length. At least one of the second light emittingelements is disposed at a position of the shortest path length.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will be described indetail based on the following figures, wherein:

FIG. 1A is a cross-sectional view illustrating an example of theconfiguration of alight emitting element array according to a firstexemplary embodiment;

FIG. 1B is a plan view of the light emitting element array;

FIG. 2 is an explanatory view of a path length from a pad of a lightemitting unit in a light emitting area according to the first exemplaryembodiment;

FIGS. 3A to 3D are images of light emitting units illustrating theresults of an ESD withstand voltage test of a light emitting elementarray according to a comparative example;

FIG. 3E is a plan view illustrating the arrangement of the respectivelight emitting units of the light emitting element array according tothe comparative example;

FIG. 4A is a graph illustrating optical output characteristics of theresult of the ESD withstand voltage test of the light emitting elementarrays according to comparative examples;

FIGS. 4B to 4E are graphs each illustrating spectrum characteristics;

FIGS. 5A to 5F are cross-sectional views illustrating an example of amethod of manufacturing the light emitting element array according tothe first exemplary embodiment;

FIGS. 6A to 6F are plan views each illustrating an example of thearrangement of light emitting units in a light emitting area accordingto a modification of the first exemplary embodiment;

FIG. 7A is a plan view illustrating an example of the configuration ofan optical transmission device according to a second exemplaryembodiment;

FIG. 7B is a cross-sectional view of the optical transmission device;

FIG. 8A is a side view illustrating a coupled state of a light emittingelement array and an optical fiber in the optical transmission deviceaccording to the second exemplary embodiment; and

FIG. 8B is a plan view illustrating the coupled state.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments for carrying out the presentinvention will be described in detail with reference to the drawings.

First Exemplary Embodiment

An example of the configuration of a light emitting element array 10according to the present exemplary embodiment will be described withreference to FIGS. 1A and 1B. An example in which a vertical cavitysurface emitting laser (VCSEL) array is applied to a light emittingelement array according to the present exemplary embodiment will bedescribed below. FIG. 1A is a cross-sectional view of the light emittingelement array 10 according to the present exemplary embodiment. FIG. 1Bis a plan view of the light emitting element array 10. Thecross-sectional view illustrated in FIG. 1A is a cross-sectional viewtaken along the line A-A′ in the plan view illustrated in FIG. 1B. Asone example, the light emitting element array 10 is used in an opticaltransmitter of an optical transmission device. Light emitted from thelight emitting element array 10 is coupled to an optical transmissionpath of an optical fiber or the like. In the present exemplaryembodiment, plural VCSELs are mainly used in order to secure theredundancy of light emission from the optical transmitter. That is, eachof the light emitting elements constituting the light emitting elementarray according to the exemplary embodiment has a rating capable ofoutputting the amount of light required to perform communication as asingle light emitting element, and the light emitting element array isconfigured with the plural light emitting elements connected in parallelto each other so that even if one of the light emitting elements isdamaged, normal communication is still maintained. With thisconfiguration, redundancy is secured. It should be noted that it is notalways necessary for each light emitting element to be capable ofemitting the amount of light required to perform communication as asingle light emitting element.

As illustrated in FIG. 1A, the light emitting element array 10 is formedinto a stacked structure. The stacked structure includes an n-sideelectrode wiring 30, an n-type lower distributed Bragg reflector (DBR)14 formed on an n-type gallium arsenide (GaAs) substrate 12, an activelayer area 16, an oxide confinement layer 26, a p-type upper DBR 18, aninterlayer insulating film 20, and a p-side electrode wiring 22.

As illustrated in FIG. 1B, the light emitting element array 10 includesa light emitting area 40 and a p-side electrode pad 28.

The light emitting area 40 is an area configured as a VCSEL arrayincluding multiple light emitting units. In the present exemplaryembodiment, for example, four light emitting units 50-1, 50-2, 50-3, and50-4 (which may be collectively referred to as a “light emitting unit50” below) formed in mesa shapes are provided. The area of each lightemitting unit excluding an emission aperture is covered with the p-sideelectrode wiring 22, so that the respective light emitting units areelectrically connected in parallel to each other. It should be notedthat in the light emitting element array 10 according to the presentexemplary embodiment, a light emission aperture of at least one of themultiple light emitting units 50 is shielded by the p-side electrodewiring 22 so that no light is emitted through the light emissionaperture. In the light emitting element array 10, an emission apertureof the light emitting unit 50-3 is shielded, whereas emission aperturesof the light emitting units 50-1, 50-2 and 50-4 are not shielded. Inother words, when power is supplied to the light emitting element array10, light is emitted substantially simultaneously from the lightemitting units 50-1, 50-2 and 50-4, except for the light emitting unit50-3. Hereinafter, a mesa which has a shielded light emission aperturewill also be referred to as the “light emitting unit 50”, forconvenience.

The p-side electrode pad 28 is a pad that is configured as a portion ofthe p-side electrode wiring 22. The p-side electrode pad 28 is a pad towhich a positive electrode of a power supply is connected when the powersupply for supplying a current to the light emitting area 40 via thep-side electrode wiring 22 is connected. In addition, a negativeelectrode of the power supply is connected to the n-side electrodewiring 30 which is formed on the back surface of the substrate 12.

Assuming that the oscillation wavelength of the light emitting elementarray 10 is λ and the refractive index of a medium (semiconductor layer)is n, the n-type lower DBR 14 formed on the substrate 12 is amultilayered-film reflector formed by alternately and repeatedlystacking two semiconductor layers which have a film thickness of 0.25λ/nand have different refractive indices.

The active layer area 16 formed on the lower DBR 14 is an area thatgenerates light to be emitted from a light emitting unit 50. The activelayer area 16 includes a lower spacer 114, a quantum well active layer116, and an upper spacer 118 which are formed in this order on the lowerDBR 14 (see FIGS. 5A to 5F).

The quantum well active layer 116 according to the present exemplaryembodiment may be configured with, for example, barrier layers of fourGaAs layers and quantum well layers of three InGaAs layers each providedbetween the adjacent GaAs layers. In addition, the lower spacer 114 andthe upper spacer 118 are respectively disposed between the quantum wellactive layer 116 and the lower DBR 14 and between the quantum wellactive layer 116 and the upper DBR 18. With this configuration, thelower spacer 114 and the upper spacer 118 have a function of adjustingthe length of a resonator and serve as clad layers for confiningcarriers.

The p-type oxide confinement layer 26, provided on the active layer area16, is a current confinement layer. The p-type oxide confinement layer26 includes a non-oxidized area 26 a and an oxidized area 26 b. Thecurrent flowing from the p-side electrode pad 28 toward the n-sideelectrode wiring 30 is narrowed (confined) by the non-oxidized area 26a.

The upper DBR 18, formed on the oxide confinement layer 26, is amultilayered-film reflector formed by alternately and repeatedlystacking two semiconductor layers which have a film thickness of 0.25λ/nand have different refractive indices.

An emission surface protection layer 24 is formed on the upper DBR 18 ofthe light emitting units 50-1, 50-2 and 50-4, to protect a lightemission surface. The emission surface protection layer 24 is formed,for example, by depositing a silicon nitride film. On the other hand,the upper DBR 18 of the light emitting unit 50-3 is covered with thep-side electrode wiring 22. A portion of the light emitting unit 50-3that corresponds to the emission surface protection layer 24 of thelight emitting units 50-1, 50-2 and 50-4 is a shield portion 32 formedof a metal film. That is, the light emission aperture of the lightemitting unit 50-3 is shielded from light.

As illustrated in FIGS. 1A and 1B, the interlayer insulating film 20 asan inorganic insulating film is deposited around a semiconductor layerincluding the mesa of the light emitting unit 50. The interlayerinsulating film 20 is disposed below the p-side electrode wiring 22 andthe p-side electrode pad 28. The interlayer insulating film 20 accordingto the present exemplary embodiment is formed of, for example, a siliconnitride film (SiN film). It should be noted that the material of theinterlayer insulating film 20 is not limited to the silicon nitridefilm. The material of the interlayer insulating film 20 may be, forexample, a silicon oxide film (SiO₂ film) or a silicon oxynitride film(SiON film).

As illustrated in FIG. 1A, in the light emitting unit 50-1, the p-sideelectrode wiring 22 is connected to the upper DBR 18 through an openingin the interlayer insulating film 20 (the same applies to the lightemitting units 50-2 and 50-4). A contact layer 124 (see FIGS. 5A to 5F)for connection with the p-side electrode wiring 22 is provided in theuppermost layer of the upper DBR 18. One end side of the p-sideelectrode wiring 22 is connected to the upper DBR 18 via the contactlayer 124 and forms an ohmic contact with the upper DBR 18.

Meanwhile, the VCSEL which constitute the light emitting unit 50 (thelight emitting units 50-1, 50-2 and 50-4) of the light emitting elementarray 10 extracts a laser output in a direction perpendicular to thesubstrate and further facilitates array formation by two-dimensionalintegration. Thus, VCSELs are suitably used, for example, as a lightsource for optical communication.

The VCSEL includes a pair of distributed Bragg reflectors (the lower DBR14 and the upper DBR 18) provided on a semiconductor substrate (thesubstrate 12) and an active layer area (the active layer area 16) formedbetween the pair of distributed Bragg reflectors. The optical module isconfigured such that current is injected into the active layer area byelectrodes (the p-side electrode wiring 22 and the n-side electrodewiring 30) provided on the opposite sides of the distributed Braggreflectors, laser oscillation is generated perpendicularly to thesubstrate surface, and oscillated light is emitted from the top of anelement (the surface side of the emission surface protection layer 24).

In addition, an oxide confinement layer (the oxide confinement layer 26)which is formed by oxidizing a semiconductor layer containing Al in thecomposition thereof is provided in terms of, for example, low thresholdcurrent and controllability of a transverse mode. In order to oxidizethe semiconductor layer containing Al, the element is etched into a mesashape and is subjected to an oxidation treatment. Thereafter, themesa-shaped side surface exposed by the etching processing and theetched semiconductor surface are generally covered with an insulatingmaterial such as a silicon nitride film or a silicon oxide film.

Here, not only the light emitting element array 10 according to thepresent exemplary embodiment but also semiconductor elements might bedamaged by electrostatic discharge (ESD). That is, discharge currentflows into the semiconductor element due to a surge applied from theoutside or the like, which might damage the semiconductor element bylocal heat generation and electric field concentration. A surge causedby ESD generally reaches an internal circuit of a semiconductor elementthrough an input/output terminal (input/output pad) or a power supplyterminal (power supply pad) of the semiconductor element, which mightdamage the internal circuit.

Therefore, how much resistance a manufactured semiconductor element hasagainst a surge due to ESD may be known in advance. As a test for this,an ESD withstand voltage test is known. The ESD withstand voltage testapplies a high voltage pulse which simulates a surge due to ESD to asemiconductor element via a terminal (pad) to know the damaged state ofthe semiconductor element or the like. In the present exemplaryembodiment, “surge” means at least one of surge current or surgevoltage. In addition, “damage (damaged)” includes a state where a lightemitting element does not completely emit light and a state where theperformance of a light emitting element is deteriorated due to a surge,such as a reduction in the amount of light.

The inventors carried out the ESD withstand voltage test for lightemitting element arrays and inspected the state of individual lightemitting units in the light emitting element arrays. The inspectionresults reveals that the degree of damage due to ESD depends on the pathlength on a wiring from a terminal to each of the plural light emittingunits along the path of current.

A path length from a terminal to a light emitting unit 50 will bedescribed with reference to FIG. 2. FIG. 2 is a view illustrating thelight emitting area including light emitting units 50-1, 50-2, 50-3 and50-4, the p-side electrode pad 28, and the p-side electrode wiring 22,which are extracted from the light emitting element array 10 illustratedin FIGS. 1A and 1B. In the present exemplary embodiment, the lightemitting units 50-1, 50-2, 50-3 and 50-4 are electrically connected inparallel. In FIG. 2, a current I flows from the p-side electrode pad 28to the light emitting area 40. Apart of the current I is supplied toeach of the light emitting units 50-1, 50-2, 50-3 and 50-4. In thiscase, the path of the current from the p-side electrode pad 28 to thelight emitting unit 50-3 through the p-side electrode wiring 22 isdefined as a path length L1. From this definition, the path length forthe light emitting unit 50-2 is defined as L2, the path length for thelight emitting unit 50-4 is defined as L2, and the path length for thelight emitting unit 50-1 is defined as L3. In the example illustrated inFIG. 2, the light emitting units 50-2 and 50-4 are equidistant from thep-side electrode pad 28. In the example illustrated in FIG. 2, amagnitude relationship between the path lengths L1, L2 and L3 is set tobe L1<L2<L3. That is, the path length L1 of the light emitting unit 50-3is the shortest path length, and the path length L3 of the lightemitting unit 50-1 is the longest path length. In the light emittingelement array according to the present exemplary embodiment, asillustrated in FIG. 2, the light emitting unit 50-3 disposed at theshortest path length is shielded.

The ESD withstand voltage test for light emitting element arrays carriedout by the inventors reveals that even if plural light emitting units 50are disposed to have small spacing therebetween, such as about 50 μm,stress is not equally applied to all the light emitting units 50, and alight emitting unit 50 having a shorter path length, that is, the lightemitting unit 50 which is disposed more upstream on the path of thedriving current is more easily damaged. Meanwhile, as will be describedlater, the results also reveals that the wavelength spectrum of theoptical output varies when the light emitting unit 50 is damaged by ESDor the like. That is, unless any measure is taken based on a position onthe path of current from a power supply in the light emitting elementarray, there is a possibility that the wavelength spectrum of theoptical output varies due to ESD or the like, thereby deteriorating thetransmission quality of a transmission device using the light emittingelement array.

More specifically, in the light emitting element array, when a surgevoltage such as ESD is applied, the respective light emitting unitsdeteriorate to different degrees depending on the intensity of stressapplied to the respective light emitting units. In addition, the greaterthe deterioration of the light emitting unit, the greater the amount ofvariation in emitted wavelength compared to that before deterioration.That is, when a surge voltage such as ESD is applied, the wavelengthspectrum in each light emitting unit varies from the initial state. As aresult, the uniformity of wavelength spectra among the multiple lightemitting units is deteriorated, which has a possibility of adverselyaffecting the signal quality of optical transmission compared to beforedeterioration. In particular, the following limitation may be imposed onan optical transmission device which will be described later. Thestandard core diameter of a multi-mode fiber is as small as 100 μm orless (50 μm or 62.5 μm) or about 100 μm or less. Thus, when a multi-modefiber is used as, for example, an optical transmission path forcommunication, the number of light emitting units 50 may often belimited to, for example, 5 or less (about 2 to 5) in order to causelight from the plural light emitting units 50 connected in parallel toenter the core of the multi-mode fiber. Therefore, in such an opticaltransmission device, variation in wavelength spectrum in one lightemitting unit 50 has a great effect on variation in the wavelengthspectrum in the entire light emitting element array.

In the exemplary embodiment of the present invention, at least oneemission aperture of the light emitting unit 50 disposed at the positionof the shortest path length (hereinafter may be referred to as a“shortest light emitting unit 50S”) is shielded, and at least oneemission aperture of the light emitting unit 50 disposed at the positionof the longest path length (hereinafter may be referred to as a “longestlight emitting unit 50L”) is not shielded. That is, the shortest lightemitting unit 50S, which is the most easily damaged by ESD or the like,is configured so as not to output light. Therefore, even if the shortestlight emitting unit 50S is damaged due to application of ESD or the likethereto, variation in the wavelength spectrum of the total opticaloutput from the light emitting element array is reduced, compared to acase where the shortest light emitting unit 50S is not shielded. Inaddition, the longest light emitting unit 50L, which is the mostdifficult to be damaged by ESD or the like, is not shielded. Therefore,compared to a case in which the longest light emitting unit 50L isshielded and the shortest light emitting unit 50S is not shielded,variation in the wavelength spectrum of the total optical output fromthe light emitting element array when ESD or the like is applied isreduced. In addition, in particular, if a multi-mode fiber is used as anoptical transmission path, variation in the wavelength spectrum of theentire light emitting element array is effectively reduced. In addition,it may be determined whether to shield light emitting units other thanthe shortest light emitting unit 50S and the longest light emitting unit50L, based on the optical output power or the like which is required fora device (such as an optical transmission device) to which the lightemitting device array is applied.

Next, the results of the ESD damage test performed for a light emittingelement array according to a comparative example will be described withreference to FIGS. 3A to 3E and FIGS. 4A to 4E. The light emittingelement array according to the comparative example used in this testincludes a light emitting area 90 according to the comparative exampleillustrated in FIG. 3E. That is, the light emitting area 90 includeslight emitting units 50-1, 50-2, 50-3 and 50-4 having the samestructure. A p-side electrode wiring 22 extends from the light emittingarea 90 to a lower part of the sheet of the figure and is connected to ap-side electrode pad 28 (not illustrated). The path length of the lightemitting unit 50-1 is equal to the path length of the light emittingunit 50-2. The path length of the light emitting unit 50-3 is equal tothe path length of the light emitting unit 50-4. The path lengths of thelight emitting units 50-1 and 50-2 are shorter than those of the lightemitting units 50-3 and 50-4. In other words, two shortest lightemitting units 50S (50-1 and 50-2) are provided in the light emittingarea 90. In addition, the distance between the centers of the respectivelight emitting units is set to about 50 μm. The distance from the centerposition of the p-side electrode pad 28 (not illustrated) to theposition of center of gravity of the plural light emitting units 50-1 to50-4 (the center position of the light emitting area 90) is set to about160 μm.

FIGS. 3A to 3D illustrate the light emitting state of the light emittingarea 90 after the voltage of the ESD test is applied to the lightemitting element array according to the comparative example via thep-side electrode pad 28. The arrangement of the light emitting units 50in FIGS. 3A to 3D is the same as that in FIG. 3E. FIG. 3A illustratesthe light emitting state when the applied voltage is 0V (that is, theinitial state). FIG. 3B illustrates the light emitting state when theapplied voltage is 150V. FIG. 3C illustrates the light emitting statewhen the applied voltage is 200V. FIG. 3D illustrates the light emittingstate when the applied voltage is 250V.

As illustrated in FIG. 3B, there is almost no change in the lightemitting state of the light emitting unit 50 when the applied voltage is150V. As illustrated in FIGS. 3C and 3D, when the applied voltages are200V and 250V, the amount of light of the light emitting units 50-1 and50-2 is reduced. From these results, it can be seen that a lightemitting unit 50 having a shorter path length is more easily damaged andthat if plural light emitting units 50 have the shortest path length,they are easily damaged simultaneously.

FIGS. 4A to 4E illustrate the results obtained by inspecting the opticaloutput characteristics of light emitting element arrays according tofour comparative examples after the voltages are applied. FIG. 4Aillustrates the inspection result of optical outputs P. FIG. 4Billustrates the inspection result of the spectrum of output light of thelight emitting unit 50-4. FIG. 4C illustrates the inspection result ofthe spectrum of output light of the light emitting unit 50-3. FIG. 4Dillustrates the inspection result of the spectrum of output light of thelight emitting unit 50-1. FIG. 4E illustrates the inspection result ofthe spectrum of output light of the light emitting unit 50-2.

It can be seen from FIG. 4A that although there is almost no change fromthe initial state when the applied voltage is 150V (in FIG. 4A, curvesfor the applied voltages of 0V and 150V overlap each other), thecharacteristics of the optical output P with respect to the current Ideteriorate when the applied voltage is increased to 200V and 250V.

It can be seen from FIGS. 4B and 4C that the light emitting units 50-4and 50-3 have substantially no change in spectrum even if the appliedvoltage is increased. On the other hand, it can be seen from FIGS. 4Dand 4E that the wavelengths of the light from the light emitting units50-1 and 50-2 are shifted to short wavelengths and then disappear as theapplied voltage increases.

That is, it can be seen from the test results of FIGS. 4B to 4E thatwhen a failure occurs in a part of the light emitting units 50 of thelight emitting area 90, the uniformity among the spectralcharacteristics of the plural light emitting units 50 is lost. When sucha phenomenon occurs, for example, there is a risk that a deteriorationin transmission quality may occur in an optical transmission devicewhich will be described later. From this point, the number of lightemitting units that are simultaneously damaged may be reduced even if asurge is applied, and a measure may be taken for the light emitting unitdepending on the position thereof in the light emitting area.

Next, a method of manufacturing the light emitting element array 10according to the present exemplary embodiment will be described withreference to FIGS. 5A to 5F. The light emitting element array 10includes four light emitting units 50 as illustrated in FIG. 1B, and thelight emitting units 50 are manufactured by the same manufacturingprocess, except for the formation of the shield portion 32. It should benoted that in FIGS. 5A to 5F, components having the same names butdifferent reference numerals from those in FIGS. 1A and 1B have the samefunctions.

First, as illustrated in FIG. 5A, an n-type lower DBR 112, an activelayer area 130, a p-type AlAs layer 120, a p-type upper DBR 122, and thecontact layer 124 are sequentially stacked on a substrate 110 formed ofn-type GaAs by the metal organic chemical vapor deposition (MOCVD)method. The n-type lower DBR 112 is formed by stacking 30 pairs of AlAsand GaAs so that each film thickness becomes one quarter of theintra-medium wavelength λ′ (=λ/n). The n-type lower DBR 112 has acarrier concentration of 1×10¹⁸ cm⁻³. The active layer area 130 includesthe lower spacer 114 formed of undoped Al_(0.22)Ga_(0.78)As, the undopedquantum well active Layer 116 (configured with three InGaAs quantum welllayers having a film thickness of 80 nm and four GaAs barrier layershaving a film thickness of 150 nm), and the upper spacer 118 formed ofundoped Al_(0.22)Ga_(0.78)As. A film thickness of the active layer area130 is the intra-medium wavelength λ′. The p-type AlAs layer 120 has acarrier concentration of 1×10¹⁸ cm⁻³ and a film thickness of one quarterof the intra-medium wavelength λ′. The p-type upper DBR 122 is formed bystacking 22 pairs of Al_(0.9)Ga_(0.1)As and GaAs so that each filmthickness becomes one quarter of the intra-medium wavelength λ′. Thep-type upper DBR 122 has a carrier concentration of 1×10¹⁸ cm⁻³. A totalfilm thickness of the p-type upper DBR 122 is about 2 μm. The contactlayer 124, is formed of p-type GaAs, has a carrier concentration of1×10¹⁹ cm⁻³ and a film thickness of the intra-medium wavelength λ′.

The film formation is successively performed by using trimethylgallium,trimethylaluminum, trimethylindium, and arsine as a raw gas,cyclopentadinium magnesium as a p-type dopant material, and silane as ann-type dopant material, setting the substrate temperature to 750° C.during the film growth, and sequentially changing the raw gases withoutbreaking a vacuum.

Next, as illustrated in FIG. 5B, the stacked films are etched to anintermediate portion of the lower DBR 112 to form a mesa 126, and theside surface of the AlAs layer 120 is exposed. In order to process themesa shape, a resist mask R is formed on a crystal growth layer byphotolithography, and reactive ion etching using carbon tetrachloride asan etching gas is performed.

Thereafter, the resist mask R is removed. As illustrated in FIG. 5C,only the AlAs layer 120 is oxidized from the lateral side by water vaporin a furnace at about 400° C. so that the resistance thereof isincreased. As a result, the AlAs layer 120 is formed into an oxidizedarea 132 and a non-oxidized area 120 a. The diameter of the non-oxidizedarea 120 a is, for example, about 3 μm. This non-oxidized area 120 aserves as a current injection area.

Thereafter, as illustrated in FIGS. 5D and 5E, an interlayer insulatingfilm 134 formed of SiN is vapor-deposited thereon, except for the uppersurface of the mesa 126. A p-side electrode wiring 136 formed of Ti/Auis formed thereon, except for an emission aperture 140, using the resistmask R. At this time, the shield portion 32 is formed on the mesa 126,which is to be formed into the shielded light emitting unit 50, usingTi/Au without forming the resist mask R. In addition, Au/Ge is depositedas the n-side electrode wiring 138 on the back surface of the substrate110. In this way, the light emitting element array 10 illustrated inFIG. 5F is completed. The mode in which the current confinementstructure is formed by oxidation is described by way of an example inthe present exemplary embodiment. It should be noted that the exemplaryembodiments are not limited thereto. For example, a current confinementstructure may be formed by ion implantation.

<Arrangement of Light Emitting Units>

Next, the arrangement of the light emitting units 50 in the lightemitting area 40 of the light emitting element array according to thepresent exemplary embodiment will be described with reference to FIGS.6A to 6F. In addition to the arrangement of the light emitting units 50illustrated in FIGS. 1A and 1B, various arrangements are applicable tothe light emitting element array according to the present exemplaryembodiment. In FIGS. 6A to 6F, the shielded light emitting units areindicated as shielded light emitting units 50B, and an non-shieldedlight emitting units are indicated as the light emitting units 50.

Arrangement 1: As illustrated in FIGS. 6B, 6D and 6E, multiple lightemitting units 50 are provided.

With this arrangement, the amount of light emitted from one lightemitting unit is reduced when the required optical output P is constant,compared to a case where only one light emitting unit 50 is provided.Therefore, the service life of each of the light emitting units 50 overtime is increased.

Arrangement 2: As illustrated in FIGS. 6B, 6C, 6D and 6F, each of theshielded light emitting units 50B is disposed at a position other thanthe position of the longest path length.

With this arrangement, since light is emitted only from the lightemitting units 50, each of which is the least affected by ESD, variationin wavelength spectrum is further reduced.

Arrangement 3: As illustrated in FIGS. 6C and 6D, multiple shieldedlight emitting units 50B are disposed at the positions of the shortestpath length.

With this arrangement, all of the light emitting units that are the mostaffected by ESD are shielded. Therefore, variation in wavelengthspectrum is further reduced, compared to a case where none of the lightemitting units is shielded.

Arrangement 4: As illustrated in FIGS. 6B and 6D, multiple lightemitting units 50 are disposed at the positions of the longest pathlength.

With this arrangement, the amount of light emitted from one lightemitting unit 50 is reduced while reducing variation in wavelengthspectrum by causing the multiple light emitting units 50 that are theleast affected by ESD. Therefore, the service life over time isincreased.

Arrangement 5: The shielded light emitting unit 50B is shifted to aposition that is closer to the p-side electrode pad (terminal) than thelight emitting unit 50 on the wiring.

With this arrangement, the influence of heat (thermal interference) ofthe shielded light emitting unit 50B on the light emitting unit 50 isreduced.

Second Exemplary Embodiment

An optical transmission device 200 according to the present exemplaryembodiment will be described with reference to FIGS. 7A and 7B and FIGS.8A and 8B. An optical transmission device 200 is a device thatconstitutes an optical transmitter of a communication apparatus whichperforms mutual optical communication via an optical fiber. The opticaltransmission device 200 is equipped with the light emitting elementarray 10 according to the above-described exemplary embodiment.

FIG. 7A is a plan view of the optical transmission device 200, and FIG.7B is a cross-sectional view taken along the line B-B′ illustrated inFIG. 7A. As illustrated in FIGS. 7A and 7B, the optical transmissiondevice 200 includes a light emitting element array 10, a monitor photodiode (PD) 62, a sub-mount 214, and a package on which these componentsare mounted. The package of the optical transmission device 200 includesa stem 202, a cap 204, a cathode terminal 216, anode terminals 218 and219 (in FIG. 7B, the anode terminal 219 is invisible because it ishidden behind the anode terminal 218), and a cathode terminal 220.

The sub-mount 214 is a substrate on which the light emitting elementarray 10, the monitor PD 62, and the like are mounted. The sub-mount 214is configured with, for example, a semiconductor substrate. In additionto the light emitting element array 10 and the monitor PD 62,semiconductor elements which constitute a drive unit of the lightemitting element array 10 and the like and required passive componentssuch as a resistor and a capacitor may be mounted on the sub-mount 214.In addition, an n-side wiring 212 made of a metal film or the like isformed on the surface side of the sub-mount 214 on which the lightemitting element array 10 and the like are mounted. The n-side electrodewiring 30 of the light emitting element array 10 is connected to then-side wiring 212.

The stem 202 is a metal base on which the sub-mount 214 is mounted. Thestem 202 holds the cathode terminal 216, the anode terminals 218 and219, and the cathode terminal 220. The cathode terminal 216 and theanode terminals 218 and 219 are held on the stem via a requiredinsulator. The cathode terminal 220 is directly brazed to (has the samepotential as) the stem 202.

As illustrated in FIG. 7A, the p-side electrode pad 28 of the lightemitting element array 10 is connected to an anode electrode 208 by abonding wire and is connected to the outside (for example, a drive powersupply) via an anode terminal 218. Meanwhile, the n-side electrodewiring 30 of the light emitting element array 10 is connected to acathode electrode 210 via the n-side wiring 212 and a bonding wiring,and is connected to the outside (for example, the drive power supply)via the cathode terminal 216.

The monitor PD 62 is a monitor for monitoring the amount of light of theoptical output P from the light emitting unit 50 of the light emittingelement array 10 when the light emitting element array 10 is driven andcontrolled. That is, for example, when the light emitting element array10 is driven and controlled by automatic power control (APC), a monitorcurrent Im corresponding to the optical output P is generated and issupplied to an APC control circuit. Of course, the driving controlmethod of the light emitting element array 10 is not limited to the APCmethod. The driving control method of the light emitting element array10 may be a constant-current driving method, a constant-voltage drivingmethod, or the like.

The anode of the monitor PD 62 is connected to an anode electrode 206via a bonding wire, and is connected to the outside (for example, thedrive power supply) via the anode terminal 219. Meanwhile, the cathodeof the monitor PD 62 is connected to the stem 202 by a bonding wire andis connected to the outside (for example, the drive power supply) viathe cathode terminal 220.

The cap 204 seals a semiconductor element or the like mounted on thesub-mount 214 in an airtight manner. The cap 204 of the presentexemplary embodiment is formed of a metal. A cap 204 is formed with anopening so as to allow the optical output P from the light emittingelement array 10 to pass therethrough. A partial reflection mirror 222is attached to the opening. Most of the optical output P passes throughthe partial reflection mirror 222 and is output to the outside (anoptical fiber which will be described later in the present exemplaryembodiment). However, a part of the light (about 10% as an example) isreflected by the partial reflection mirror 222 and is incident on themonitor PD 62 as monitor light Pm. The monitor light Pm generates theabove-described monitor current Im.

Next, the coupling between the light emitting element array 10 and anoptical fiber 300 will be described with reference to FIGS. 8A and 8B.FIG. 8A is a cross-sectional view illustrating the coupled state of thelight emitting element array 10 and the optical fiber 300. FIG. 8B is aplan view illustrating the coupled state. As the optical fiber 300according to the present exemplary embodiment, for example, a singlemode fiber, a multi-mode fiber, or a plastic fiber may be used withoutparticular limitation thereto. In the present exemplary embodiment, amulti-mode fiber which has a core diameter of 100 μm or less (50 μm or62.5 μm) or about 100 μm or less is used.

As illustrated in FIG. 8A, the optical fiber 300 includes a core 302 anda clad 304. As illustrated in FIGS. 8A and 8B, the light emitting unit50 of the light emitting element array 10 is disposed such that theoptical output P enters the core 302 of the optical fiber 300. In thepresent exemplary embodiment, no lens is used for the coupling betweenthe light emitting element array 10 and the optical fiber 300. It shouldbe noted that the disclosure is not limited thereto. The light emittingelement array 10 and the optical fiber 300 may be coupled to each otherusing a lens.

Here, the following arrangement may be applicable as the arrangement ofthe light emitting units 50 in the light emitting element array 10mounted in the optical transmission device 200. This arrangement isgiven by way of example as one arrangement according to the aboveexemplary embodiment.

Arrangement 6: The light emitting unit(s) 50 are disposed at positionsthat are closer to the center of the core 302 than the shielded lightemitting unit(s) 50B.

With this arrangement, the influence of heat (thermal interference) ofthe shielded light emitting unit(s) 50B on the light emitting unit(s) 50is reduced.

The optical transmission device 200, which is mounted on the can-shapedpackage, has been described in the above-described exemplary embodimentby way of example. It should be noted that the present disclosure is notlimited thereto. An optical transmission device may be mounted on a flatpackage.

Third Exemplary Embodiment

Hereinafter, an optical transmission device according to the presentexemplary embodiment will be described. In the light emitting elementarray 10 and the optical transmission device 200 according to the aboveexemplary embodiments, in order to prevent deterioration in theuniformity of spectra of the optical output P from the light emittingelement array 10, at least one emission aperture of the shortest lightemitting unit 50S is shielded and at least one emission aperture of thelongest light emitting unit SOL is not shielded so that no light isemitted from the shortest light emitting unit 50S. In the presentexemplary embodiment, instead of shielding the shortest light emittingunit 50S, the shortest light emitting unit 50S is arranged so that thelight emitted from the shortest light emitting unit 50S is not coupledto the optical fiber. An optical transmission device 200 a according tothe present exemplary embodiment is identical to the opticaltransmission device 200, except for the arrangement of the shortestlight emitting unit 50S. Therefore, description of the opticaltransmission device 200 a according to the present exemplary embodimentwill refer to FIGS. 7A and 7B and FIGS. 8A and 8B if necessary, andillustration of the optical transmission device 200 a will be omitted.

In the present exemplary embodiment, the shortest light emitting unit50S is disposed as follows. That is, the longest light emitting unit 50Lis disposed such that the light emitted from the longest light emittingunit 50L enters the optical fiber 300 and the shortest light emittingunit 50S is disposed such that the optical axis of the light emittedfrom the shortest light emitting unit 50S deviates from the optical axisof the optical fiber 300. The expression “the shortest light emittingunit 50S is disposed such that the optical axis of the light emittedtherefrom deviates from the optical axis of the optical fiber 300” meansthat for example, in FIG. 8A, the position of the light emitting unit 50is shifted in a plane that is parallel to a plane perpendicular to thecore so as to prevent the optical output P emitted from the lightemitting unit 50 from entering the core 302. In addition, it may bedetermined whether to shift the optical axis of the light emitting unitsother than the shortest light emitting unit 50S and the longest lightemitting unit 50L, based on, for example, the optical output powerrequired in the optical transmission apparatus 200 a or the like.

As described above, deterioration in the uniformity of the wavelengthspectrum is reduced by the optical transmission device according to thepresent exemplary embodiment.

In addition, the light emitting element array in which light emittingunits are monolithically formed has been described in the respectiveexemplary embodiments. It should be noted that the disclosure is notlimited thereto. Individual (discrete) light emitting elements asrespective light emitting units may be used.

The foregoing description of the exemplary embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in the art. Theembodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with the various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

What is claimed is:
 1. An optical transmission device comprising: alight emitting element array comprising a plurality of light emittingelements connected in parallel to each other by a wiring connected to aterminal that supplies a current, each of the light emitting elementsbeing disposed at a position of a predetermined path length along a pathof the current flowing from the terminal through the wiring, theplurality of light emitting elements including, in a mixed form, one ormore first light emitting elements and one or more second light emittingelements, wherein at least one of the first light emitting elements isdisposed at a position of the longest path length of the path andconfigured to emit light to a medium, wherein at least one of the secondlight emitting elements is disposed at a position of the shortest pathlength of the path and is configured to prevent light emitted by said atleast one of the second light emitting elements from entering themedium, wherein the plurality of light emitting elements are verticalcavity surface emitting laser (VCSEL) elements, wherein the one or morefirst light emitting elements each has a non-shielded light emissionaperture and the one or more second light emitting elements each has ashielded light emission aperture, wherein the medium is an opticaltransmission path configured to transmit light, and the light emittingelement array is disposed such that light emitted from the lightemitting element array enters the optical transmission path, wherein theoptical transmission path includes a core configured to propagate thelight, and wherein the one or more first light emitting elements aredisposed at positions that are closer to a center of the core than theone or more second light emitting elements.
 2. The optical transmissiondevice according to claim 1, wherein wherein the plurality of lightemitting elements includes three or more light emitting elements, andwherein the one or more first light emitting elements include aplurality of the first light emitting elements.
 3. The opticaltransmission device according to claim 1, wherein wherein the pluralityof light emitting elements includes three or more light emittingelements, and wherein the one or more second light emitting elements aredisposed at positions other than the position of the longest pathlength.
 4. The optical transmission device according to claim 1, whereina plurality of the second light emitting elements are disposed at thepositions of the shortest path length.
 5. The optical transmissiondevice according to claim 1, wherein a plurality of the first lightemitting elements are disposed at the positions of the longest pathlength.
 6. The optical transmission device according to claim 1, whereinall of the one or more second light emitting elements are disposed atpositions that are closer to the terminal than all of the one or morefirst light emitting elements on the wiring.
 7. The optical transmissiondevice according to claim 1, wherein the first light emitting element isdisposed such that the light emitted from the first light emittingelement enters the optical transmission path, and wherein the secondlight emitting element is disposed such that an optical axis of thelight emitted from the second light emitting element deviates from anoptical axis of the optical transmission path.
 8. An opticaltransmission device comprising: a light emitting element arraycomprising a plurality of light emitting elements connected in parallelto each other by a wiring connected to a terminal that supplies acurrent, each of the light emitting elements being disposed at aposition of a predetermined path length along a path of the currentflowing from the terminal through the wiring, the plurality of lightemitting elements including, in a mixed form, one or more first lightemitting elements and one or more second light emitting elements,wherein at least one of the first light emitting elements is disposed ata position of the longest path length of the path and configured to emitlight to a medium, and wherein at least one of the second light emittingelements is disposed at a position of the shortest path length of thepath and is configured to prevent light emitted by said at least one ofthe second light emitting elements from entering the medium, wherein theplurality of light emitting elements are vertical cavity surfaceemitting laser (VCSEL) elements, wherein the one or more first lightemitting elements each has a non-shielded light emission aperture andthe one or more second light emitting elements each has a shielded lightemission aperture, wherein the medium is an optical transmission pathconfigured to transmit light, and the light emitting element array isdisposed such that light emitted from the light emitting element arrayenters the optical transmission path, and wherein the opticaltransmission path is a multi-mode fiber having a core diameter of about100 μm or less.
 9. The optical transmission device according to claim 1,wherein the optical transmission path is a multi-mode fiber having acore diameter of about 100 μm or less.
 10. The optical transmissiondevice according to claim 7, wherein the optical transmission path is amulti-mode fiber having a core diameter of about 100 μm or less.